1. Field of the Invention
This invention relates to an earphone comprising a drum air power system, and specifically, to a technique for improving the performance of sound reproduction in the low pitch tone range of, for example, an inner ear headphone.
2. Brief Description of the Related Arts
Hearing aids equipped with a balanced armature driver unit have been proposed previously (see, for example, Japanese Patent Laid-Open No. 2007-7499). The balanced armature driver unit has a basic structure, including a coil, magnets, and a vibrator, that is similar to the structure of a dynamic driver unit. In the balanced armature driver unit, however, only the vibrator is driven. The balanced armature driver unit thus attains high efficiency acoustic conversion and enables sound reproduction with high sensitivity and high accuracy, even in response to weak signals. The size of the balanced armature driver unit can be reduced to the size of a grain of rice. Inner ear headphones equipped with a balanced armature driver unit are described in Japanese Patent Laid-Open No. 2009-49757.
A small driver unit, such as the balanced armature driver unit, however, has a high minimum resonance frequency and produces a low sound pressure. Small driver units emit low-pitched tones with a weak absolute sound volume. Therefore, small driver units are not suitable for sound reproduction in the low pitch tone range and are incapable of actualizing rich acoustic quality. In many prior art descriptions of small driver units, sound in the low pitch tone range is reproduced by vibrations of a rubber material. These prior art techniques, however, produce only dull and heavy acoustical output.